Microsystems technology is a rapidly developing field. One range of Microsystems applications is the combination of actuators or sensors and electronic circuits onto a single integrated circuit device. Currently, integrated circuit (IC) sensors and actuators, such as piezoresistive pressure sensors, are manufactured using a surface micromachining process. Surface micromachining has advantages over the previous bulk micromachining process of fabricating sensors and actuators because it permits smaller devices and may by integrated with other circuits on an IC.
One IC technology is complementary metal oxide semiconductor (CMOS) technology. This well-known IC fabrication process provides a very high cell-density (e.g., many circuits per unit area), is relatively inexpensive, and yields reliable circuits. It would be desirable to fabricate piezoresistive pressure sensors, for example, using CMOS technology.
Several techniques for integrating sensors and signal conditioning circuits onto a single IC are known. FIGS. 1A and 1B are a cross-sectional view and top view, respectively, of a conventional IC piezoresistive pressure sensor. As seen in FIGS. 1A and 1B, a piezoresistive pressure sensor 100 includes a silicon membrane (or diaphragm) 102 micromachined onto a silicon substrate 104. Resistors 106 are diffused into the membrane 102 at certain locations. A cavity 108 is provided on the underside of the substrate 104 from which pressure is applied to the membrane 102. Pressure on the silicon membrane stresses the membrane, which affects the resistance of the resistors 106. The change in resistance 106 is detected by external circuitry 110, and the change of resistance is used to determine the pressure applied to the membrane 102.
In order for a piezoresistive sensor described above to operate, membranes having different thicknesses are used for sensing different pressure ranges. Thicknesses typically range from 10 microns to 30 microns. For example, it may be preferable to use a 15 micron thick membrane to sense 15 psi pressure. Thus, in order to provide an accurate piezoresistive pressure sensor for a particular pressure range, the membrane must be fabricated to a precise thickness.
Two techniques typically used for fabricating a micromachined piezoresistive sensor are (1) surface micromachined silicon etching and wafer bonding; and (2) using epitaxial wafers for electrochemical etch stopping. Neither technique is compatible with CMOS fabrication technology. For example, the second technique achieves a precise membrane thickness using an electrochemical etch stop method. FIGS. 2A and 2B illustrate the fabrication of a conventional IC piezoresistive pressure sensor using this second technique. As seen in FIG. 2A, a p-type silicon substrate 202 has an n-type epitaxial layer 204 grown on it. As seen in FIG. 2B, the substrate is etched through etch windows 206 using alkaline etchants (such as KOH) in an etch bath. At the same time, a positive bias 208 is applied to the epitaxial layer 204. The positive bias does not affect the p-type substrate due to the n-p reverse bias condition. The n-type epitaxial layer (which forms the membrane) is prevented from being etched due to the passivating positive bias. This allows a precise membrane thickness to be obtained. CMOS processes, however, do not allow for n-type epitaxial layers on the substrate. Resistors may then be formed in the membrane using p-type diffusion doping.
Therefore it is an object of the present invention to provide a process for manufacturing IC sensors that is compatible with conventional IC processes, such as CMOS.